Colloidal electrolytes

ABSTRACT

A colloidal electrolyte for an electrochemical device. The colloidal electrolyte includes a liquid electrolyte selected from liquid organic electrolytes, or liquid inorganic electrolytes free of sulfuric acid; and a ceramic particle phase dispersed in the liquid electrolyte, wherein the colloidal electrolyte has increased conductivity in the electrochemical device compared to the conductivity of the liquid electrolyte alone. The colloidal electrolytes will suppress flammability and flowability.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.60/372,334 filed Apr. 12, 2002.

BACKGROUND OF THE INVENTION

The present invention relates to electrolytes used in electrochemicaldevices, and more particularly, to colloidal electrolytes havingincreased conductivity in the electrochemical device compared to theconductivity of the liquid electrolyte alone.

The use of liquid electrolytes in electrochemical devices presents anumber of difficulties. First, the electrolytes are often corrosive, andthey can be difficult to contain. For example, lead storage batteriesuse sulfuric acid as the electrolyte. If the sulfuric acid corrodesthrough the container, it can create environmental problems. As aresult, attempts have been made to immobilize liquid electrolytes. Theseefforts have had varying degrees of success. In addition, many organicelectrolytes are flammable.

Furthermore, the performance of electrochemical devices can suffer underlow or high temperature conditions. For example, cold temperaturesreduce the conductivity of the current solid or liquid electrolytes usedin lithium batteries. At temperatures less than −20° C., some liquidelectrolytes freeze, resulting in a major drop in ionic conductivity andcapacity. In addition, cold temperatures slow down the charge transferreaction kinetics at the electrode-electrolyte interfaces. Thus, whenliquid electrolytes are used in an electrochemical device such as alithium ion battery, the rate capability and performance of the batteryis slowed as a result of the effect of the cold temperature on theliquid electrolytes. Under high temperature conditions, liquidelectrolytes can suffer from instability and degradation, which isbelieved to be due primarily to the decomposition of the lithium salt,such as LiPF₆.

Attempts have been made to decrease the effect of cold temperatures onliquid electrolytes and, consequently, to improve the performance of theelectrochemical devices containing them. For example, insulatingblankets, heaters, and phase change materials have been developed tokeep liquid electrolytes warm in lithium ion and other batteries.Although these efforts have helped to improve the conductivity of theelectrolytes and the charge transfer reaction kinetics of lithium ionbatteries under cold temperature conditions, they increase the cost perkilowatt-hour, and they lower the energy and power densities of thebattery.

Thus, there is a need for an electrolyte composition which can maintaingood conductivity under a wide range of temperatures.

SUMMARY OF THE INVENTION

The present invention meets this need by providing a colloidalelectrolyte for an electrochemical device. Electrochemical devicesinclude, but are not limited to, fuel cells, batteries of various types,including lithium batteries, electrochemical sensors, and electrochromicdevices. The colloidal electrolyte provides improved charge transferreaction kinetics and rate capability. In addition, the colloidalelectrolyte exhibits reduced thermal degradation when subjected to hightemperatures.

The colloidal electrolyte of the present invention includes a liquidelectrolyte selected from liquid organic electrolytes, or liquidinorganic electrolytes free of sulfuric acid; and a ceramic particlephase dispersed in the liquid electrolyte, wherein the colloidalelectrolyte has increased conductivity in the electrochemical devicecompared to the conductivity of the liquid electrolyte.

When the colloidal electrolyte is to be used in a lithium battery, theliquid electrolyte further comprises a lithium salt. Suitable lithiumsalts include, but are not limited to, lithium halides and complexfluorides, such as lithium hexafluorophosphate, or lithiumtetrafluoroborate.

Suitable liquid organic electrolytes include, but are not limited to,ethylene carbonate, diethyl carbonate, polypropylene carbonate, ormixtures thereof.

Suitable liquid inorganic electrolytes free of sulfuric acid include,but are not limited to, potassium hydroxide, phosphoric acid, or moltencarbonate.

The average size of the ceramic particles is generally in the range ofabout 1 to about 10,000 nm. Suitable ceramic particles include, but arenot limited to, MgO, ZnO, SrO, BaO, CaO, ZrO₂, Al₂₀₃, SiO₂, SiC, Si₃N₄,BN, BaTiO₃, or mixtures thereof.

The liquid electrolyte is generally present in an amount of betweenabout 3 to about 98 wt % and the ceramic particle phase is generallypresent in an amount of between about 97 to about 2 wt %.

Another aspect of the invention is an electrochemical device containingthe claimed colloidal electrolyte. Still another aspect of the inventionis a lithium rechargeable battery containing the claimed colloidalelectrolyte.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic comparing the conductivity of a liquid electrolyteand a colloidal electrolyte of the present invention.

FIG. 2 is a graph showing the complex impedance plot of a liquidelectrolyte at 20° C.

FIG. 3 is a graph showing the complex impedance plot of a liquidelectrolyte at −20° C.

FIG. 4 is a graph showing the complex impedance plot of a colloidalelectrolyte at 20° C.

FIG. 5 is a graph comparing the resistivity of a liquid electrolyte anda colloidal electrolyte containing BaTiO₃ particles.

FIG. 6 is a graph comparing the resistivity of a liquid electrolyte anda colloidal electrolyte containing Al₂O₃ particles.

FIG. 7 is a graph comparing the conductivity of a liquid electrolyte anda colloidal electrolyte containing BaTiO₃ particles.

FIG. 8 is a graph comparing the conductivity of a separator materialimpregnated with a liquid electrolyte, and a separator materialimpregnated with a colloidal electrolyte containing Al₂O₃ particles.

DETAILED DESCRIPTION OF THE INVENTION

The presence of the ceramic particles in the colloidal electrolytes ofthe present invention improves the mechanical and thermal properties ofthe electrolyte. The colloidal electrolytes display increasedconductivity in the electrochemical device compared to the conductivityof the liquid electrolyte alone. In addition, the colloidal electrolytehas increased viscosity which reduces the flowability of theelectrolyte, making it easier to contain. The presence of the ceramicparticles in organic electrolytes also reduces the flammability of theorganic electrolytes. Finally, the diffusion and flow of gases throughthe liquid electrolyte, which would be detrimental to theelectrochemical performance of a fuel cell, is are reduced by thepresence of the ceramic particles.

FIG. 1 is a schematic showing the effect of the colloidal electrolyte ofthe present invention on conductivity. The conductivity of the liquidelectrolyte drops slowly as the temperature is reduced from T₁ to T₂ (T₂is near the freezing point of the liquid electrolyte). It then dropsquickly after reaching temperature T₂, yielding a useful operating rangefor the liquid electrolyte of T₁–T₂.

In contrast, the conductivity of the colloidal electrolyte of thepresent invention drops slowly over a much wider temperature range, fromT₁ to T₃. Thus, the useful operating range of the colloidal electrolyteof the present invention is T₁–T₃, which is greater than the operatingrange for the liquid electrolyte alone, T₁–T₂. The increase in theuseful operating range and the level of improvement in conductivity varybased on the particular liquid electrolyte and ceramic particles used.

The colloidal electrolyte of the present invention includes a ceramicparticle phase dispersed in a liquid electrolyte. The liquid electrolytecan be liquid organic electrolytes, or liquid inorganic electrolytesfree of sulfuric acid.

Liquid organic electrolytes which can be used in the present inventioninclude, but are not limited to, lithium salts dissolved in organicsolvents. Suitable organic solvents include, but are not limited to,ethylene carbonate, diethyl carbonate, polypropylene carbonate,sulfolane, dimethylsulfoxide, acetonitrile, tetrahydrofuran, or mixturesthereof. These liquid organic electrolytes are useful for lithiumbatteries.

Liquid inorganic electrolytes free of sulfuric acid can also be used.Liquid inorganic electrolytes include, but are not limited to, potassiumhydroxide, phosphoric acid, or molten carbonate. These liquid inorganicelectrolytes can be used in fuel cells. When employed alone as anelectrolyte, potassium hydroxide can be used at temperatures in therange of from about 100° C. to about 250° C., phosphoric acid can beused in the range of from about 150° C. to about 250° C., and moltencarbonate can be used in the range of from about 500° C. to about 700°C. The operating ranges can be increased and/or the conductivity can beimproved by including ceramic particles in the liquid inorganicelectrolyte.

Suitable ceramic particles include, but are not limited to, MgO, ZnO,SrO, BaO, CaO, ZrO₂, Al₂O₃, SiO₂, SiC, Si₃N₄, BN, BaTiO₃, or mixturesthereof. The average size of the ceramic particles is generally in therange of about 1 to about 10,000 nm. It is generally desirable to useparticles at the lower end of the range, such as about 1 to about 5,000nm, or about 1 to about 1,000 nm, or about 1 to about 500 nm, or about 1to about 100 nm. The particles can be surface treated to increaseconductivity further, if desired.

The liquid electrolyte is generally present in an amount of betweenabout 3 to about 98 wt % and the ceramic particle phase is generallypresent in an amount of between about 97 to about 2 wt %, typicallyabout 30 to about 90 wt % liquid electrolyte and about 10 to about 70 wt% ceramic particle phase, and more typically about 50 to 90 wt % liquidelectrolyte and about 10 to about 50 wt % ceramic particle phase.

The colloidal electrolyte can be made by any suitable process. Theparticles can be mixed in the liquid electrolyte to form the colloidalelectrolyte. The mixing can be done manually or using a mechanical,ultrasonic, or other type of mixer. The colloidal electrolytes can alsobe prepared by an in situ process in which particles are precipitated ina compatible liquid phase.

When the colloidal electrolyte is to be used in a lithium battery, theliquid electrolyte further comprises a lithium salt. Suitable lithiumsalts include, but are not limited to, lithium halides and complexfluorides, such as lithium hexaflurophosphate, or lithiumtetrafluoroborate.

Several electrolytes used in lithium batteries maintain their fluiditydown to −40° C. However, their conductivity decreases rapidly below −20°C. because ionic mobility is greatly reduced due to the increase insolution viscosity. This view is supported by the observation that below−20° C., conductivity is not very sensitive to the dielectric constantof the solvents, but is highly influenced by the solution viscosity.Therefore, it would be expected that the addition of nanosize ceramicparticles to form the colloidal electrolytes would increase the solutionviscosity and consequently lower the conductivity of the electrolytes.However, it was unexpectedly found that the presence of the ceramicparticles actually increased the conductivity of the colloidalelectrolyte compared to the conductivity of the electrolyte alone.

The invention takes advantage of the fact that the dielectric constant,k, of a nanosize ceramic is inversely proportional to temperature, i.e.,k increases as temperature decreases. In some cases, such as withferroelectric ceramics (e.g., BiTiO₃), there is a rapid increase in k asthe temperature is reduced. The presence of such a nanosize ceramic inthe vicinity of the lithium salt will enhance its dissociation.Consequently, more lithium ions will become available to carry charges.The effect is similar to the use of high dielectric constant solvents informulating electrolytes for lithium ion batteries.

While not wishing to be bound to a particular theory, it is believedthat degradation of an electrolyte in a lithium-ion battery attemperatures above 50° C. occurs through the following reaction:${{{LiPF}_{6}\underset{\_}{\longrightarrow}{LiF}} + {PF}_{5}};{K = {\frac{\lbrack{LiF}\rbrack\left\lbrack {PF}_{5} \right\rbrack}{\left\lbrack {LiPF}_{6} \right\rbrack}.}}$

The colloidal electrolyte of the present invention is described in moredetail by way of the following examples, which are intended to beillustrative of the invention, but are not intended to be limiting inscope.

EXAMPLE 1

The conductivity of a liquid electrolyte was compared with theconductivity of a colloidal electrolyte of the present invention. Acommercially available liquid electrolyte comprising a 1:1 solvent blendof ethylene carbonate and diethyl carbonate with a molar solution oflithium hexafluorophosphate as the lithium salt was used. The density ofthe liquid electrolyte was about 1.27 g/cc.

The colloidal electrolyte was made by mixing the liquid electrolytedescribed above with about 10 wt % of BaTiO₃ (average particle size ofabout 1 μm). The mixture was stirred for about 15 minutes.

The conductivity of the liquid electrolyte and the colloidal electrolytewas determined using a three electrode cell. The electrodes were made ofstainless steel with a surface area of about 1.99 cm². The gap betweenthe electrodes was about 0.02 cm.

The resistance of the liquid electrolyte and the colloidal electrolytewas measured. The resistance of the liquid electrolyte at a temperatureof 20° C. and −20° C. are shown in FIGS. 2 and 3, respectively. Theconductivity of the electrolyte was calculated from the resistance usingthe geometric parameters of the cell. The conductivity of the liquidelectrolyte was 0.86 mScm⁻¹ at −20° C., while the conductivity of theliquid electrolyte at 20° C. was 1.46 mScm⁻¹.

The resistance of the colloidal electrolyte is shown in FIG. 4. Theconductivity of the colloidal electrolyte containing BaTiO₃ particles at20° C. was 12.7 mScm⁻¹, which is about nine times greater than theconductivity of the liquid electrolyte without the BaTiO₃.

The electrolyte-electrode interface resistance for the liquidelectrolyte was approximately 100 kΩ at −20° C. and 1.1 Ω at 20° C. Acomparison of the resistivity of the liquid electrolyte and thecolloidal electrolyte containing 10 wt % of BaTiO₃ is shown in FIG. 5.

EXAMPLE 2

A liquid electrolyte comprising a 1:1 solvent blend of ethylenecarbonate, and polypropylene carbonate with a molar concentration oflithium hexafluorophosphate was made. The density of the liquidelectrolyte was about 1.25 g/cc.

The colloidal electrolyte was formed by mixing the liquid electrolytedescribed above with 10 wt % Al₂O₃ (average particle size of about 24nm).

The resistance of the liquid electrolyte and the colloidal electrolytewas measured using a two electrode cell. The electrodes were made ofstainless steel with a surface area of about 0.44 cm² and a gap of 0.96cm. The electrodes were inserted through one port in a glass container.The electrolyte was poured into a second port, submerging theelectrodes. The ports were then sealed, and the test was run. Theresults are shown in FIG. 6.

The conductivity was then calculated. The table below summarizes theresults, including the temperature at which the conductivity wasmeasured, the conductivity of the liquid electrolyte, the conductivityof the colloidal electrolyte, and the ratio of the conductivities.

Temperature Liquid Electrolyte (σ₁) Colloidal Electrolyte (σ_(c)) (° C.)(mScm⁻¹) (mScm⁻¹) σ_(c)/σ₁ −40 0.33 1.15 3.48 −30 0.82 2.48 3.02 −201.64 4.64 2.83 −10 2.66 7.52 2.83 0 4.07 11.70 2.87 10 6.06 16.50 2.7220 7.79 21.60 2.77 30 10.40 27.30 2.63 40 12.80 33.60 2.63 50 15.6039.70 2.54The results show that the colloidal electrolyte has increasedconductivity compared with the conductivity of the liquid electrolytealone. The degree of conductivity enhancement varies with temperature,and it is more pronounced at lower temperatures.

EXAMPLE 3

A liquid electrolyte comprising a 1:1 solvent blend of ethylenecarbonate, and polypropylene carbonate with a molar concentration oflithium hexafluorophosphate was made. The density of the liquidelectrolyte was about 1.25 g/cc.

The colloidal electrolyte was formed by mixing the liquid electrolytedescribed above with 10 wt % BaTiO₃ (average particle size of about 1μm).

The resistance of the liquid electrolyte and the colloidal electrolytewas measured using a two electrode cell. The electrodes were made ofstainless steel with a surface area of about 0.44 cm² and a gap of 0.96cm. The electrodes were inserted through one port in a glass container.The electrolyte was poured into a second port, submerging theelectrodes. The ports were then sealed, and the test was run. Theconductivity was calculated, and the results are shown in FIG. 7. Thecolloidal electrolyte containing BaTiO₃ showed improved conductivitycompared to the ethylene carbonate, polypropylene carbonate, lithiumhexafluorophosphate liquid electrolyte.

EXAMPLE 4

A liquid electrolyte comprising a 1:1 solvent blend of ethylenecarbonate, and polypropylene carbonate with a molar concentration oflithium tetrafluoroborate was made. The density of the liquidelectrolyte was about 1.25 g/cc.

Two colloidal electrolytes were formed by mixing the liquid electrolytedescribed above with 10 wt % BaTiO₃ (average particle size of about 1μm), and with 10 wt % Al₂O₃ (average particle size of about 24 nm).

A microporous separator material was soaked in the electrolyte (liquidor colloidal) for several hours and then used. The microporous separatormaterial was a copolymer of tetrafluoroethylene and ethylene in the formof a 100 μm thick film.

The resistance of the liquid electrolyte and the colloidal electrolyteswas measured using a two electrode cell. The electrodes were made ofstainless steel with a surface area of about 1.23 cm². The separatormaterial was sandwiched between the two electrodes, and the electrodeswere manually tightened to obtain good contact. The conductivity wascalculated, and the results are shown in FIG. 8. Both colloidalelectrolytes showed increased conductivity over the ethylene carbonate,polypropylene carbonate, lithium tetrafluoroborate liquid electrolytealone. The colloidal electrolyte containing Al₂O₃ had slightly betterconductivity than the one containing BaTiO₃.

While the invention has been described by reference to certainembodiments, it should be understood that numerous changes could be madewithout departing from the scope of the invention defined in theappended claims. Accordingly, it is intended that the invention not belimited to the disclosed embodiments, but that it have the full scopepermitted by the language of the following claims.

1. A liquid colloidal electrolyte for an electrochemical devicecomprising: a liquid electrolyte selected from liquid organicelectrolytes, or liquid inorganic electrolytes free of sulfuric acid;and a ceramic particle phase dispersed in the liquid electrolyte,combined to form the liquid colloidal electrolyte; wherein the liquidcolloidal electrolyte has increased conductivity in the electrochemicaldevice compared to the conductivity of the liquid electrolyte alone. 2.The liquid colloidal electrolyte of claim 1 wherein the liquidelectrolyte is a liquid organic electrolyte.
 3. The liquid colloidalelectrolyte of claim 2 wherein the liquid organic electrolyte comprisesa lithium salt dissolved in a liquid organic solvent.
 4. The liquidcolloidal electrolyte of claim 3 wherein the lithium salt is selectedfrom lithium halides or complex fluorides.
 5. The liquid colloidalelectrolyte of claim 4 wherein the lithium halides or complex fluoridesare selected from lithium hexafluorophosphate or lithiumtetrafluoroborate.
 6. The liquid colloidal electrolyte of claim 3wherein the liquid organic solvent is selected from ethylene carbonate,diethyl carbonate, polypropylene carbonate, sulfolane,dimethylsulfoxide, acetonitrile, tetrahydrofuran, or mixtures thereof.7. The liquid colloidal electrolyte of claim 1 wherein the liquidelectrolyte is a liquid inorganic material free of sulfuric acid.
 8. Theliquid colloidal electrolyte of claim 7 wherein the liquid inorganicmaterial free of sulfuric acid is selected from potassium hydroxide,phosphoric acid, or molten carbonate.
 9. The liquid colloidalelectrolyte of claim 1 wherein an average size of the ceramic particlesis in the range of about 1 to about 10,000 nm.
 10. The liquid colloidalelectrolyte of claim 1 wherein the ceramic particles are selected fromMgO, ZnO, SrO, BaO, CaO, ZrO₂, Al₂O₃, SiO₂, SiC, Si₃N₄, BN, BaTiO₃, ormixtures thereof.
 11. The liquid colloidal electrolyte of claim 1wherein the liquid electrolyte is present in an amount of between about3 to about 98 wt % and the ceramic particle phase is present in anamount of between about 97 to about 2 wt %.
 12. A liquid colloidalelectrolyte for an electrochemical device comprising: a liquid organicelectrolyte containing a lithium salt; and a ceramic particle phasedispersed in the liquid electrolyte, combined to form the liquidcolloidal electrolyte; wherein the liquid colloidal electrolyte hasincreased conductivity in the electrochemical device compared to theconductivity of the liquid organic electrolyte alone.
 13. The liquidcolloidal electrolyte of claim 12, wherein the liquid organicelectrolyte comprises a lithium salt dissolved in an organic solvent.14. The liquid colloidal electrolyte of claim 13 wherein the organicsolvent is selected from ethylene carbonate, diethyl carbonate,polypropylene carbonate, sulfolane, dimethylsulfoxide, acetonitrile,tetrahydrofuran, or mixtures thereof.
 15. The liquid colloidalelectrolyte of claim 13 wherein the lithium salt is selected fromlithium halides and complex fluorides.
 16. The liquid colloidalelectrolyte of claim 15 wherein the lithium halides and complexfluorides are selected from lithium hexafluorophosphate, or lithiumtetrafluoroborate.
 17. The liquid colloidal electrolyte of claim 12wherein an average size of the ceramic particles is in the range ofabout 1 to about 10,000 nm.
 18. The liquid colloidal electrolyte ofclaim 12 wherein the ceramic particles are selected from MgO, ZnO, SrO,BaO, CaO, ZrO₂, Al₂O₃, SiO₂, SiC, Si₃N₄, BN, BaTiO₃, or mixturesthereof.
 19. The liquid colloidal electrolyte of claim 12 wherein theliquid organic electrolyte is present in an amount of between about 3 toabout 98 wt % and the particle phase is present in an amount of betweenabout 97 to about 2 wt %.
 20. An electrochemical device containing theliquid colloidal electrolyte of claim
 1. 21. A lithium rechargeablebattery containing the liquid colloidal electrolyte of claim
 2. 22. Anelectrochemical device containing the liquid colloidal electrolyte ofclaim
 12. 23. A lithium rechargeable battery containing the liquidcolloidal electrolyte of claim
 12. 24. The liquid colloidal electrolyteof claim 1 wherein the liquid electrolyte is present in an amount ofbetween about 30 to about 90 wt % and the ceramic particle phase ispresent in an amount of between about 10 to about 70 wt %.
 25. Theliquid colloidal electrolyte of claim 1 wherein the liquid electrolyteis present in an amount of between about 50 to about 90 wt % and theceramic particle phase is present in an amount of between about 10 toabout 50 wt %.
 26. The liquid colloidal electrolyte of claim 12 whereinthe liquid electrolyte is present in an amount of between about 30 toabout 90 wt % and the ceramic particle phase is present in an amount ofbetween about 10 to about 70 wt %.
 27. The liquid colloidal electrolyteof claim 12 wherein the liquid electrolyte is present in an amount ofbetween about 50 to about 90 wt % and the ceramic particle phase ispresent in an amount of between about 10 to about 50 wt %.